Heatwaves have large societal impacts that are roughly proportional to the hottest surface air temperature (SAT) during the event – the heatwave intensity. Future changes in heatwave intensity depend not only on the long-term, time-mean warming but also on changes in the variance in SAT; in particular, increases in SAT variance at daily timescales would amplify future heatwave intensity. The work will analyze; (i) the physical mechanisms responsible for daily variance of SAT across the globe in observations; (ii) potential biases in SAT variance in global coupled climate models, and the physical mechanism responsible for those biases and; (iii) robust mechanisms of future changes in SAT variance and their impact on heatwave intensity. Specifically, the work develops a framework for decomposing daily SAT variance into contributions from the movement of energy by atmospheric winds, solar radiation reaching the surface, and changes in soil moisture using observational datasets. Preliminary work suggests that SAT variance in the middle latitudes is governed primarily by the import of tropical air into a region and is thus set by the strength and duration of wind patterns. The impact of model wind biases on SAT variance is diagnosed using targeted model simulations in which the winds are nudged to match observed winds. The wind nudging framework is used in conjunction with a surface energy balance model to understand changes in SAT variance in future projections from an ensembl